The present invention relates to devices with electronic components, such as an antenna with an inductor.
Radio Frequency Identification (RFID) tags allow for the remote identification of objects through the use of radio waves (electromagnetic radiation in a radio frequency spectrum). RFID tags may, for example, be placed on products in a warehouse such that an inventory of existing products in the warehouse may be performed electronically merely by interrogating each RFID tag on the products. RFID tags typically include at least one IC (integrated circuit) which acts as a radio receiver and transmitter and an antenna. The antenna may include an inductor, such as an inductive loop in the antenna's layout or design. The one purpose of the inductor is to cancel the reactance cause by the intrinsic capacitance of the IC. When the inductor is at its optimum value, the antenna can couple the maximum amount of RF energy to and from the IC.
One method of manufacturing an RFID tag is to place one or more ICs in a carrier substrate (a first substrate) and attach the carrier substrate to an antenna substrate (a second substrate). Both substrates may be flexible and the antenna pattern, with an inductor, may be printed or otherwise formed on the antenna substrate. Examples of such a method of manufacture is described in U.S. patent application Ser. No. 09/872,985, filed May 31, 2001, which issued as U.S. Pat. No. 6,606,247 on Aug. 12, 2003 and is hereby incorporated herein by reference. FIG. 3 of U.S. Pat. No. 6,606,247 shows an RFID tag 300 that is manufactured by combining and interconnecting a carrier strap 301, which includes at least one IC, and a receiving substrate 310 which includes an antenna 311. An IC in the carrier strap 301 may be placed into a receptor region of the carrier strap 301 by a fluidic self-assembly (FSA) process, such as a process described in U.S. Pat. No. 5,545,291 which is hereby incorporated herein by reference. Alternatively, the IC may be placed into the receptor region by a robotic pick and place operation.
This manufacturing process requires that there be some registration of the carrier strap 301 relative to the receiving substrate so that the electrical contacts on the carrier strap 301 matingly engage the corresponding electrical contacts of the receiving substrate 310. This registration requirement tends to complicate the manufacturing process and tends to increase the effective cost of the manufacturing process. This registration can become important when an inductor is used in the antenna pattern on the receiving substrate.
FIGS. 1A and 1B show two examples of RFID tags which are manufactured by combining a carrier strap, which is carrier strap 10 or carrier strap 14 in FIGS. 1A and 1B respectively, with an antenna or receiving substrate which includes an antenna pattern that has one inductor. The antenna pattern 11 of FIG. 1A includes an inductive loop (or inductor) 12, and the antenna pattern 15 of FIG. 1B includes an inductive loop 16. The placement of the carrier strap (e.g. straps 10 or 14) onto the respective receiving substrate will effect the value of the inductance. For example, if the carrier strap is positioned closer to the left side of the pattern in FIG. 1A, then the length and area of the inductive loop are increased relative to an inductive loop formed by a strap which is positioned closer to the right side of the pattern. The desired inductance of the loops is a function of the operating frequency. For example, at an operating frequency of 2.45 GHz, a desired inductor value may be about 7.2 times smaller than a desired inductor value at 915 MHz (based on the calculation of (2.45/0.915)^2). This small inductor is very sensitive to the strap's location since the strap is part of the inductor loop. The smaller the loop the greater the variation in the inductance value with a given change in the strap position.